NOx adsorber aftertreatment system for internal combustion engines

ABSTRACT

The present invention provides for an NOx adsorber aftertreatment system for internal combustion engines which utilizes a parallel arrangement of an adsorber catalyst and a bypass. The exhaust flow from the engine is routed through the adsorber during lean operation. At a predetermined regeneration time (for example, when the adsorber catalyst is 20% full), the exhaust gas flow is reduced through the parallel leg that contains the adsorber catalyst to be regenerated (e.g., 20% through the catalyst leg, 80% of the flow to the bypass leg). A quantity of hydrocarbon is injected into the reduced-flow catalyst leg in order to make the mixture rich. Since the flow has been reduced in this leg, only a small fraction of the amount of hydrocarbon that would have been required to make the mixture rich during full flow is required. This will result in a substantial reduction in the fuel penalty incurred for regeneration of the adsorber catalyst. Once the leg has been regenerated, the exhaust flow is switched to flow 100% through the adsorber leg.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to internal combustion enginesand, more particularly, to an NOx adsorber aftertreatment system forinternal combustion engines.

BACKGROUND OF THE INVENTION

As environmental concerns have led to increasingly strict regulation ofengine emissions by governmental agencies, reduction of nitrogen-oxygencompounds (NOx) in exhaust emissions from internal combustion engineshas become increasingly important. Current indications are that thistrend will continue.

Future emission levels of diesel engines will have to be reduced inorder to meet Environmental Protection Agency (EPA) regulated levels. Inthe past, the emission levels of U.S. diesel engines have been regulatedaccording to the EPA using the Federal Test Procedure (FTP) cycle, witha subset of more restrictive emission standards for California via theCalifornia Air Resources Board (CARB). For example, the Tier II emissionstandards, which are being considered for 2004, are 50% lower than theTier I standards. Car and light truck emissions are measured over theFIT 75 test and expressed in gm/mi. Proposed Ultra-Low Emissions Vehicle(ULTEV) emission levels for light-duty vehicles up to model year 2004are 0.2 gm/mi NOx and 0.08 gm/mi particulate matter (PM). Beginning withthe 2004 model year, all light-duty Low Emission Vehicles (LEVs) andULEVs in California would have to meet a 0.05 gm/mi NOx standard to bephased in over a three year period. In addition to the NOx standard, afull useful life PM standard of 0.01 gm/mi would also have to be met.

Traditional methods of in-cylinder emission reduction techniques such asexhaust gas recirculation (EGR) and injection rate shaping by themselveswill not be able to achieve these low emission levels required by thestandard. Aftertreatment technologies will have to be used, and willhave to be further developed in order to meet the future low emissionrequirements of the diesel engine.

Some promising aftertreatment technologies to meet future NOx emissionstandards include lean NOx catalysts, Selective Catalytic Reduction(SCR) catalysts, and Plasma Assisted Catalytic Reduction (PACR). Currentlean NOx catalyst technologies will result in the reduction of engineout NOx emissions in the range of 10 to 30 percent for typicalconditions. Although a promising technology, SCR catalyst systemsrequire an additional reducing agent (aqueous urea) that must be storedin a separate tank, which opens issues of effective temperature range ofstorage (to eliminate freezing) as well as distribution systems thatmust be constructed for practical use of this technology. PACR issimilar to lean NOx in terms of reduction efficiency but is moreexpensive due to plasma generator. These technologies, therefore, havelimitations which may prevent their use in achieving the new emissionsrequirements.

NOx adsorber catalysts have the potential for great NOx emissionreduction (60-90%). The NOx adsorber is one of the most promising NOxreduction technologies. During lean-bum operation of the engine, thetrap adsorbs nitrogen oxide in the form of stable nitrates. Understoiciometric or rich conditions, the nitrate is thermodynamicallyunstable and the stored nitrogen oxides are released and subsequentlycatalytically reduced. Therefore, the operation cycle alternates betweenlean and rich conditions around the catalyst. During lean operation thecatalyst stores the NOx and during rich operation the NOx is releasedand reduced to No. However, to make the conditions around the catalystrich, a significant amount of hydrocarbon (HC) needs to be injected. Theamount of HC required for reduction is only a small fraction of thetotal hydrocarbon injected, resulting in a significant fuel penalty. Ifthe HC required to make. conditions rich can be reduced, the fuelpenalty can be brought down substantially.

An additional problem is the need for a diesel oxidation catalystdownstream from the NOx adsorber. The diesel oxidation catalyst oxidizesany unburned hydrocarbon that slips through the adsorber before theexhaust gases are released to the atmosphere. The need for a dieseloxidation catalyst negatively affects system cost and system packagesize.

Furthermore, some diesel engines include a catalytic soot filter to trapthe soot generated by the engine. This soot is carcinogenic to livingbeings. Such catalytic soot filters often become clogged with thetrapped particulate matter owing to the fact that they require hightemperatures to regenerate. It is difficult to attain these hightemperatures in the engine exhaust stream at low loads.

There is therefore a need for an engine aftertreatment system employingan NOx adsorber which reduces the fuel penalty associated with thesedevices, allows for regeneration of the soot filter, even at low loads,and reduces the system cost and package size. The present invention isdirected toward meeting this need.

SUMMARY OF THE INVENTION

The present invention provides for an NOx adsorber aftertreatment systemfor internal combustion engines which utilizes a parallel arrangement ofan adsorber catalyst and a bypass. The exhaust flow from the engine isrouted through the adsorber during lean operation. At a predeterminedregeneration time (for example, when the adsorber catalyst is 20% full),the exhaust gas flow is reduced through the parallel leg that containsthe adsorber catalyst to be regenerated (e.g., 20% through the catalystleg, 80% of the flow to the bypass leg). A quantity of hydrocarbon isinjected into the reduced-flow catalyst leg in order to make the mixturerich. Since the flow has been reduced in this leg, only a small fractionof the amount of hydrocarbon that would have been required to make themixture rich during full flow is required. This will result in asubstantial reduction in the fuel penalty incurred for regeneration ofthe adsorber catalyst. Once the leg has been regenerated, the exhaustflow is switched to flow 100% through the adsorber leg.

In one embodiment, a catalytic soot filter is positioned upstream fromthe adsorber. The additional hydrocarbon used to promote regeneration isinjected into the catalytic soot filter. The catalytic soot filter, whenused in combination with the adsorber, provides more time and surfacearea for the hydrocarbon to react with the oxygen. The catalytic sootfilter will additionally reformulate some of the diesel fuel intohydrogen and carbon monoxide, which have been shown to be betterreductants than diesel fuel.

In another embodiment, a catalytic soot filter is positioned downstreamfrom the adsorber. The heat generated by the regenerating adsorber istransferred downstream to the soot filter, thereby heating the sootfilter above the temperature required for regeneration. Additionally,any hydrocarbon that slips through the adsorber is burned in thecatalytic soot filter, further raising the temperature. Such burning ofthe hydrocarbon slip in the catalytic soot filter obviates the need fora diesel oxidation catalyst, thereby reducing system cost and packagesize.

In another embodiment, a catalytic soot filter is positioned upstreamfrom the sulfur trap. The soot filter converts SO₂ to SO₃, which is morereadily trapped by the sulfur trap.

In one form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a sulfur trap having a sulfur trapinput operatively coupled to the engine exhaust and having a sulfur trapoutput, a catalytic soot filter having a soot filter input operativelycoupled to the sulfur trap output and having a soot filter output, avalve system having a valve input operatively coupled to the soot filteroutput, a first valve output and having a second valve output, anadsorber having an adsorber input operatively coupled to the first valveoutput and having an adsorber output, a bypass pathway having a bypassinput operatively coupled to the second valve output and having a bypassoutput operatively coupled to the adsorber output, and a dieseloxidation catalyst having a DOC input operatively coupled to theadsorber output and to the bypass output and having a DOC output.

In another form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a valve system having a valve inputoperatively coupled to the engine exhaust, a first valve output andhaving a second valve output, an adsorber having an adsorber inputoperatively coupled to the first valve output and having an adsorberoutput, and a bypass pathway having a bypass input operatively coupledto the second valve output and having a bypass output operativelycoupled to the adsorber output.

In another form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a valve system having a valve inputoperatively coupled to the engine exhaust, a first valve output andhaving a second valve output, a catalytic soot filter having a sootfilter input operatively coupled to the valve system output and having asoot filter output, an adsorber having an adsorber input operativelycoupled to the soot filter output and having an adsorber output, and abypass pathway having a bypass input operatively coupled to the secondvalve output and having a bypass output operatively coupled to theadsorber output.

In another form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine isdisclosed, the system comprising a valve system having a valve inputoperatively coupled to the engine exhaust, a first valve output andhaving a second valve output, an adsorber having an adsorber inputoperatively coupled to the first valve output and having an adsorberoutput, a bypass pathway having a bypass input operatively coupled tothe second valve output and having a bypass output, and a catalytic sootfilter having a soot filter input operatively coupled to the adsorberoutput and the bypass output and having a soot filter output.

In another form of the invention, an internal combustion engineaftertreatment system for treating exhaust gases exiting an engine, thesystem comprising a catalytic soot filter having a soot filter inputoperatively coupled to the engine exhaust and having a soot filteroutput, a sulfur trap having a sulfur trap input operatively coupled tothe filter output and having a sulfur trap output, a valve system havinga valve input operatively coupled to the sulfur trap output, a firstvalve output and having a second valve output, an adsorber having anadsorber input operatively coupled to the first valve output and havingan adsorber output, a bypass pathway having a bypass input operativelycoupled to the second valve output and having a bypass outputoperatively coupled to the adsorber output, and a diesel oxidationcatalyst having a DOC input operatively coupled to the adsorber outputand to the bypass output and having a DOC output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a first preferred embodimentsystem of the present invention.

FIG. 2 is a schematic block diagram of a second preferred embodimentsystem of the present invention.

FIG. 3 is a process flow diagram illustrating a preferred embodimentprocess of the present invention.

FIG. 4 is a schematic block diagram of a third preferred embodimentsystem of the present invention.

FIG. 5 is a schematic block diagram of a fourth preferred embodiment ofthe present invention.

FIG. 6 is a schematic block diagram of a fifth preferred embodiment ofthe present invention.

FIG. 7 is a schematic block diagram of a sixth preferred embodiment ofthe present invention.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, and alterations and modifications in theillustrated device, and further applications of the principles of theinvention as illustrated therein are herein contemplated as wouldnormally occur to one skilled in the art to which the invention relates.

Referring to FIG. 1, there is illustrated a schematic block diagram of afirst preferred embodiment of the present invention. The system isdesigned to remove NOx compounds from the exhaust stream of an internalcombustion engine 12, such as a diesel engine. The exhaust produced bythe engine 12 exits the exhaust manifold 14 of the engine and is passedthrough an optional sulfur trap 16. NOx adsorber catalysts are extremelysensitive to the level of sulfur in the fuel. The fuel and thelubrication oil of the engine contain sulfur and therefore sulfur-oxygencompounds (SOx) are contained in the exhaust gas. This SOx is adsorbedinto the NOx adsorber and reduces its capacity. Unlike NOx, SOx does notregenerate under rich conditions within the operating temperature rangeof the engine. Eventually the adsorber is filled up with sulfate andbecomes inactive. The optional sulfur trap 16 may therefore be used totrap SOx compounds before they reach the NOx adsorbers downstream.

The output of the sulfur trap 16 may be passed through an optionalcatalytic soot filter 18 in order to trap any diesel soot particulatematter that may be entrained in the exhaust gases. In addition totrapping diesel soot particulate matter by physical filtering, thecatalytic soot filter also acts as a flow-through oxidation catalyst bythe addition of precious metal catalysts which reduce the volatileorganic fraction of the soot material by the catalyzed oxidationreaction (e.g. C+Oxidant→CO). A sensor 20 may be placed at the output ofthe soot filter 18 in order to measure the temperature and air/fuel(A/F) ratio (lambda) of the exhaust stream. The output of the optionalsensor 20 is provided to an electronic engine control module 22.

The engine controller 22 is additionally coupled to the engine 12 forreading various engine sensor data, such as engine position sensor data,speed sensor data, air mass flow sensor data, fuel rate data, etc., asis known in the art. The engine controller 22 may further provide datato the engine 12 in order to control the operating state of the engine12, as is well known in the art.

The flow of exhaust leaving the soot filter 18 is controlled by aproportional control 3-way valve 24. As is known in the art, aproportional control 3-way valve may be used to divide the flow of a gasstream into two separate paths, wherein the percentage of the total gasflow being directed to either path is controllable. In the embodiment ofFIG. 1, the proportional control 3-way valve 24 is coupled to the enginecontroller 22 in order to control the relative proportions of exhaustgas flow routed to either output of the valve 24.

The two outputs of the valve 24 are coupled to the respective inputs ofa pair of NOx adsorbers (catalytic converters) 26 and 28. Therefore, byproviding control signals from the engine controller 22 to theproportional control 3-way valve 24, the percentage of the total exhaustflow from the engine 14 entering either the adsorber 26 or the adsorber28 may be precisely controlled. A fuel injector 30 is positioned toinject a measured quantity of fuel (hydrocarbon) into the exhaust gasflow entering the adsorber 26. Similarly, a second fuel injector 32 ispositioned to inject a quantity of fuel into the exhaust gas flowentering adsorber 28. Both injectors 30, 32 are controlled by the enginecontroller 22 and are supplied with fuel from a pump 34 supplied by thevehicle fuel tank 36. Preferably, the fuel pump 34 is a low-costdiaphragm-type fuel pump. Two igniters 38 are provided to ignite thefuel being injected by the injectors 30, 32 under the control of theengine controller 22.

Because the exhaust flow is reduced in the adsorber leg beingregenerated, the amount of reductant required to burn off the oxygenreduces. The concentration of reductant required for reduction remainsthe same, but this amount is a small fraction of the total reductantduring full exhaust flow. It will be appreciated that any flow ratiosmay be utilized during reduction and regeneration and during normalflow, even though exemplary flows are used herein for illustrativepurposes. The optimum flow ratios for any given system will depend uponthe particular system configuration.

The exhaust gases exiting the adsorbers 26 and 28 are combined togetherbefore being input to an optional diesel oxidation catalyst 40. Due tothe pulse injection of relatively large quantities of reductant(normally hydrocarbon) for short periods during regeneration of the NOxadsorbers 26, 28 of the present invention, some unburned hydrocarbon canslip through the adsorber catalyst. The use of a diesel oxidationcatalyst 40 downstream of the adsorbers 26, 28 virtually eliminateshydrocarbon emission from the tailpipe. Such catalysts contain preciousmetals in them that reduce the activation energy of hydrocarboncombustion, such that the unburned hydrocarbon is oxidized to carbondioxide and water. The exhaust gases exiting the diesel oxidationcatalyst 40 may then exit the vehicle. An optional NOx sensor 42 may beplaced between the adsorbers 26, 28 and the diesel oxidation catalyst 40in order to directly measure the NOx levels leaving the adsorbers 26 and28. The output of the optional NOx sensor 42 is provided to the enginecontroller 22.

Referring now to FIG. 2, there is illustrated a second preferredembodiment of the present invention. The second embodiment of thepresent invention is similar to the first embodiment illustrated in FIG.1, and like reference designators refer to like components. In thesecond embodiment, the proportional control 3-way valve is replaced witha pair of two-way valves 50 and 52. Valve 50 controls the flow ofexhaust gases into the adsorber 26, while valve 52 controls the flow ofexhaust gases into adsorber 28. Each of the valves 50, 52 is coupled tothe engine controller 22 for control thereby.

The valves 50, 52 may comprise either variable flow rate control valvesor may comprise valves having a fixed number of flow rate settings. Forexample, if the aftertreatment system design dictates that the relativeflow between adsorbers 26, 28 will always be 20-80 during regeneration,then the valves 50, 52 may have discrete settings that will allow theengine controller 22 to switch them between reduced flow (20%) and maxflow (80%) settings in order to achieve the desired flow reduction inone of the adsorbers 26, 28. Optionally, the valves 50, 52 may havevariably adjustable flow rates, such that the engine controller 22 caninfinitely adjust the flow percentage through each valve 50, 52 in orderto divide the exhaust flow between the adsorbers 26, 28 in any desiredproportion.

Referring now to FIG. 3, there is illustrated a preferred embodimentprocess of the present invention. The process begins at step 100, whichrepresents the steady state operation of the engine with exhaust gasflow split evenly between the adsorbers 26 and 28. At step 102, theengine controller 22 determines whether either of the adsorber 26, 28catalysts need be regenerated. The decision made at step 102 can be madeunder open-loop control, by using stored catalyst adsorption maps in theengine controller 22. These catalyst adsorption maps may bepredetermined using empirical data from laboratory tests utilizing thesame or similar engine and exhaust system. The regeneration decision atstep 102 may also be made under closed-loop control, wherein the enginecontroller 22 examines the data being produced by the NOx sensor 42which is proportional to the level of NOx being emitted at the output ofthe adsorbers 26, 28.

If step 102 determines that the adsorbers 26, 28 need to be regenerated(e.g. the adsorption efficiency has dropped to 80%), then the processcontinues at step 104 in which the flow of exhaust through the system iscontrolled such that the adsorber to be regenerated receives a reducedlevel of exhaust flow. For example, if the engine controller 22determines that adsorber 26 needs to be regenerated, then the flow ofexhaust through the adsorber 26 can be reduced to 20% of the totalexhaust flow, with the remaining 80% being routed through the adsorber28. The relative proportions of exhaust flow routed to either adsorberwill depend upon various system design parameters. The 20-80 splitdiscussed herein is for illustrative purposes only.

Control of the relative flow of exhaust gases through adsorbers 26 and28 is performed under control of the engine controller 22 (for example,based upon the engine sensor parameters being sent to the controller 22(engine position sensor, speed sensor, air mass flow sensor, fuel rate,etc.)) through operation of either the proportional control 3-way valve24 of the system of FIG. 1 or through control of the dual 2-way valves50, 52 of the system of FIG. 2, which are adjusted to achieve thecorrect predetermined exhaust flow velocity needed for regeneration ofthe aftertreatment system.

Once the correct flow velocity has been achieved through each of theadsorbers 26, 28, the process moves to step 106 in which the enginecontroller 22 determines the temperature and air/fuel ratio of theregeneration exhaust stream using the sensor 20. If the temperature ofthe exhaust stream is sufficient for regeneration of the catalysts(according to a predetermined temperature limit), then the processcontinues to step 110. If step 106 determines that the temperature ofthe regeneration exhaust stream needs to be raised, then the processcontinues at step 108 in which the engine controller 22 causes theigniter 38 to be activated in order to ensure ignition of theregeneration fuel injection.

At step 110, the fuel injector 30, 32 in the leg being regenerated isused to inject the required amount of fuel into the exhaust stream as areductant to completely regenerate the catalysts within the adsorber.The injectors 30, 32 are controlled by the engine controller 22. Theexhaust fuel injector 30, 32 is used to achieve a rich air/fuel ratio(lambda less than 1.0) in the regeneration stream. Because of thereduced amount of exhaust gas flowing through the regeneration leg, thequantity of fuel needed to be injected by the injector 30, 32 is greatlyreduced, thereby significantly reducing the fuel penalty associated withadsorber regeneration. This injected fuel will be ignited by thetemperature of the exhaust gas stream (possibly supplemented by theigniter 38) in order to facilitate regeneration of the adsorber.

Once regeneration of the leg is determined to be complete at step 112(e.g. after a predetermined amount of time has elapsed), the processcontinues at step 114, where the engine controller 22 determines if bothlegs of the system have been regenerated. If they have not, then theprocess continues at step 116, where the engine controller 22 operateseither the proportional control 3-way valve 24 or the 2-way valves 50,52 in order to route the majority of the exhaust gas flow to therecently regenerated leg and to reduce the amount of exhaust gasesflowing through the leg which is to be regenerated. The process is thenreturned to step 106 in order to regenerate the next leg. If, on theother hand, step 114 determines that both legs have been regenerated,then the process is returned to step 100 where the engine controller 22operates the proportional control 3-way valve 24 or the 2-way valves 50,52 in order to evenly split the exhaust gas flow through the adsorbers26, 28.

As detailed hereinabove, the adsorber regeneration cycle switches backand forth between the two sides of the exhaust as necessary in order tokeep the outlet exhaust stream purified of excessive emissions. It willbe appreciated that since dual exhaust streams are being utilized, theregeneration cycle of the adsorber does not necessarily have to beshort. During the entire time that the adsorber is being regenerated,the second adsorber is available for cleaning the majority of theexhaust gas stream. It should also be noted that the temperature of theregeneration exhaust gas stream may also be controlled by adjustment ofthe proportional control 3-way valve in conjunction with the igniter 38.By allowing slightly more exhaust gas to pass into the regeneration sideof the exhaust, the temperature thereof may be raised.

Besides the aforementioned advantages in adsorber regeneration, thearrangement of catalysts illustrated in FIGS. 1 and 2 of the presentinvention provides other benefits. Placing the catalytic soot filter 18before the adsorbers 26, 28 helps in multiple ways. The catalytic sootfilter 18 converts the NO in the exhaust stream to NO₂ which helps NOxstorage-in the adsorber 26, 28. The catalytic soot filter 18. alsoprevents particulate matter from clogging the adsorber system and italso helps increase the temperature of the exhaust stream in order tomake the adsorber 26, 28 more efficient.

In another embodiment, the sulfur trap 16 may be placed downstream fromthe catalytic soot filter 18. By placing the catalytic soot filter 18upstream of the sulfur trap 16, the catalytic soot filter 18 willconvert SO₂ to SO₃, which is more readily trapped by the sulfur trap 16.

Therefore, the system illustrated and described herein is effective inaddressing all legislatively-controlled emissions including NOx, SOx andhydrocarbons. The adsorbers are used for reduction of NOx levels and aremore easily regenerated than in prior art systems. The sulfur trapremoves sulfur from the exhaust, making the operation of the adsorbermore efficient and longer lasting. The catalytic soot filter trapsparticulate soot from the exhaust stream. Finally, the diesel oxidationcatalyst cleans up any leftover hydrocarbons exiting the adsorbers,thereby allowing the exhaust emitted by the system of the presentinvention to meet or exceed the requirements of the various legislativebodies.

Referring now to FIG. 4, there is illustrated a third preferredembodiment of the present invention. The third embodiment of the presentinvention is similar to the first embodiment illustrated in FIG. 1, andlike reference designators refer to like components. In the thirdembodiment, the adsorber 28 and injector 32 are replaced by a simplebypass tube 29. During lean operation of the engine 12, the entireexhaust flow is routed through the adsorber 26 under control of the3-way valve 24. As in the first preferred embodiment, when the adsorber26 efficiency falls to a predetermined level (e.g. 80% efficiency), the3-way valve 24 is adjusted to route a majority of the exhaust flowthrough the bypass tube 29. As in the first preferred embodiment, theadsorber 26 may then be regenerated by the injection of hydrocarbonthrough the fuel injector 30.

After the adsorber 26 has been regenerated, the valve 24 is adjusted toroute all of the exhaust flow through the adsorber 26. In this manner,the regeneration cycle can be switched back and forth between full flowthrough the adsorber 26 and partial adsorber bypass through the tube 29in order to keep the outlet exhaust stream purified of excessiveemissions. Since the bypass tube 29 contains no adsorber, theregeneration cycle needs to be kept short in order to keep NOx emissionsto acceptable levels.

The third embodiment system of FIG. 4 has certain advantages over thefirst embodiment system. In the first embodiment system, theregeneration operation has to e performed twice in each cycle sincethere is a catalyst mounted in each leg. Use of the third embodimentsystem therefore leads to less injections of regeneration hydrocarbonand additional fuel savings. Of course, NOx is not stored in the bypasstube 29 during regeneration, thus the system efficiency of the thirdembodiment is slightly less than for the first and second embodiments.The third embodiment, however, has the advantage of less hardware byrequiring one less adsorber, fuel injector and ignitor. The thirdembodiment also utilizes a simpler control strategy because of the needto regenerate only a single adsorber.

Referring now to FIG. 5, there is illustrated a fourth preferredembodiment of the present invention. The fourth embodiment of thepresent invention is similar to the third embodiment illustrated in FIG.4, and like reference designators refer to like components. In thefourth embodiment, the catalytic soot filter 18 is moved to a positionupstream from the adsorber 26 and downstream of the fuel injector 30.

As discussed hereinabove, catalytic soot filters 18 require hightemperatures in order to regenerate. It is difficult to attain thesehigh temperatures in the exhaust stream during low load operation of theengine 12. Under these conditions, the soot filter 18 eventually becomesclogged with soot. By placing the soot filter 18 upstream from theadsorber 26 and downstream from the fuel injector 30 as shown in thefourth embodiment, the catalytic soot filter 18 also receives theinjected hydrocarbon and is regenerated by combustion of thishydrocarbon. Placement of the catalytic soot filter 18 in this positionalso provides more time and surface area for the introduced hydrocarbonto react with oxygen, thereby more completely burning the hydrocarbon.More complete hydrocarbon combustion will possibly eliminate the needfor the diesel oxidation catalyst 40, thereby reducing exhaust systemcost and package size.

Furthermore, the catalytic soot filter 18 will reformulate some of thediesel fuel into hydrogen and carbon monoxide, which have been shown tobe better reductants than diesel fuel. This improvement in reductionwill result in more complete regeneration of the catalytic soot filter18 and adsorber 26 and/or a shorter regeneration time.

Referring now to FIG. 6, there is illustrated a fifth preferredembodiment of the present invention. The fifth embodiment of the presentinvention is similar to the third embodiment illustrated in FIG. 4, andlike reference designators refer to like components. In the fifthembodiment, the diesel oxidation catalyst 40 is removed from the systemand the catalytic soot filter 18 is positioned downstream from theadsorber 26.

As discussed hereinabove, catalytic soot filter 18 requires hightemperatures in order to regenerate. It is difficult to attain thesehigh temperatures in the exhaust stream during low load operation of theengine 12. Under these conditions, the soot filter 18 eventually becomesclogged with soot. By placing the soot filter 18 downstream from theadsorber 26 as shown in the fifth embodiment, heat generated in theadsorber 26 due to the combustion of the introduced hydrocarbon servesto raise the temperature of the catalytic soot filter 18 sufficiently toaccomplish regeneration.

Furthermore, any hydrocarbon that slips unburned through the adsorber 26will oxidize in the soot filter 18, thereby generating further heat toencourage regeneration of the soot filter 18. Because the hydrocarbonslip is oxidized in the soot filter 18, the diesel oxidation catalyst 40of the prior embodiments is no longer required. Elimination of thediesel oxidation catalyst 40 reduces the exhaust system cost and packagesize.

Referring now to FIG. 7, there is illustrated a sixth preferredembodiment of the present invention. The sixth embodiment of the presentinvention is similar to the third embodiment illustrated in FIG. 4, andlike reference designators refer to like components. In the sixthembodiment, the catalytic soot filter 18 is positioned upstream from thesulfur trap 16. Placement of the catalytic soot filter 18 in thisposition enhances the efficiency of the sulfur trap, as the soot filterconverts SO₂ to SO₃, which is more readily trapped by the sulfur trap.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and -that allchanges and modifications- that come within the spirit of the inventionare desired to be protected.

1-31. (canceled)
 32. An engine exhaust aftertreatment system comprising:parallel exhaust flow paths; a NOx aftertreatment component disposed ineach of the flow paths; a reducing agent catalyst positioned downstreamof the parallel flow paths and through which exhaust flow from theparallel flow paths is constrained to pass; a SOx aftertreatmentcomponent; and a particulate aftertreatment component; wherein thesystem regulates exhaust flow through the parallel exhaust flow pathsand is operable in a rich mode wherein the system consumes a reducedamount of reducing agent attributable to the regulation of exhaust flow.33. A system according to claim 31 wherein the reduced amount ofreducing agent is attributable to exhaust flow in a first flow pathbeing increased exhaust flow in a second flow path being decreased.